Neutropenia is a severe hematological abnormality associated with several autoimmune diseases such as Felty's syndrome and systemic lupus erythromatosus (SLE). The central hypotheses of this project is that antibodies reacting with specific neutrophil surface-antigen(s) are involved in the peripheral destruction of neutrophils in some patients with autoimmune disease. This hypothesis is based on novel findings presented in this application. First, sera from four donors with autoimmune disease associated neutropenia were shown to exhibit strong binding to the surface of neutrophils in FACS analysis. Second, specific monoclonal Fab fragment antibodies reactive with surface and/or intracellular neutrophil antigens were retrieved from phage display libraries generated from bone marrow of neutropenic patients with Felty's syndrome or SLE. The specific aims are three-fold. (1) Isolate an extended panel of neutrophil-associated antibodies from the phage display libraries generated from bone marrow of patients with autoimmune disease and neutropenia by selection on live neutrophils, neutrophil membrane preparations, and neutrophil precursor cells. (2) Define the neutrophil-associated antigens(s) recognized by selected antibodies from Aim 1 using biochemical and immunological techniques and cloning of the neutropenia-associated antigen(s) from a neutrophil cDNA library expression on the surface of phage and in lgt11. (3) Determine the pathogenesis of the cloned antibodies by generating intact human IgG1 antibodies from the cloned Fab fragments and testing for their ability to sensitize neutrophils by promoting ingestion by macrophages in vitro.